Insulated container for radioactive and like substances

ABSTRACT

A double-walled container for radioactive material having an outer metal casing and an inner wall of lead and, in the space between the walls, an insulating material which is a solid mass of a mixture of hydrated alumina and hydrated iron oxide. The solid mass of insulating material may be formed in the space by pouring a mixture of an aqueous suspension of alumina trihydrate and particulate mild steel or iron into the space and allowing the mixture to set. The insulating material is effective in removing heat from the lead shield and in preventing heat reaching the lead shield from a source external of the container.

United States Patent Bochard 1 Dec. 18, 1973 [5 INSULATED CONTAINER FOR3,569,714 3 1971 Anderson et al 250/108 R RADIOACTIVE AND LIKESUBSTANCES 3,453,160 7/1969 Darling ct all. 250/108 WS X 3,466,4449/1969 Lusk t. 250/108 WS [75] Inventor: Camille Bochard, Lyon (Rhone),

France [73] Assignee: Robatel S.L.P.l., Geneve (Rhone),

France [22] Filed: July 28. 1970 [21] Appl. No.: 59,037

[52] US. Cl. ..250/507 [51] Int. Cl. G21f l/00 [58] Field of Search250/108 R, 108 WS, 250/108 FS [56] References Cited UNITED STATESPATENTS 2,773,459 12/1956 Sechy 250/108 WS X 2,726,339 12/1955B0rst..... 250/108 WS 2,992,175 7/1961 Borst i 250/108 R X. 3,558,5261/1971 Hall et a1. 250/108 R X Primary ExaminerArchie R. BorcheltAttorney-Alexander and Dowell [57 I ABSTRACT A double-walled containerfor radioactive material having an outer metal casing and an inner wallof lead and, in the space between the walls, an insulating materialwhich is a solid mass of a mixture of hydrated alumina and hydrated ironoxide. The solid mass of insulating material may be formed in the spaceby pouring a mixture of an aqueous suspension of alumina trihydrate andparticulate mild steel or iron into the space and allowing the mixtureto set. The insulating material is effective in removing heat from thelead shield and in preventing heat reaching the lead shield from asource external of the container.

7 Claims, 2 Drawing Figures INSULATED CONTAINER FOR RADIOACTIVE AND LIKESUBSTANCES This invention is concerned with an insulated container forradioactive and like substances.

Conventional containers for radioactive and like sub stances are doublewalled, the inner wall being formed of lead to provide the necessarygamma-radiation shielding, and the outer wall being a metal casing whichprovides protection against impacts, and the space between the inner andouter walls is filled with an insulating material. It has been foundthat for completely satisfactory results the insulating material musthave apparently contradictory properties. The first requirement is thatunder normal working conditions, the insulating material must be a verygood heat conductor so as to remove heat evolved by the radioactive orlike substance contained in the container so as to prevent anyoverheating of the lead shielding. A contrary requirement, however, isthat in the event of an accidental outbreak of fire in the vicinity ofthe container, the insulating material should have very poor heatconductivity so as to act as a heat barrier and effectively reduce therisk of any accidental melting of the lead shielding. Once the outbreakof fire has finished, the insulating material should have sufficientheat conductivity to remove the heat evolved by the contents of thecontainer to substantially the same extent as in normal operation.

We have now developed an insulating material which substantially meetsthese requirements, which can be used at high temperatures, and whichhas very good mechanical strength. I

I will explain the invention with reference to the accompanying drawingwhich illustrates one practical embodiment of the invention, and willsummarize in the claims the essential features of the invention forwhich protection is desired.

In said drawing:

FIG. 1 is a horizontal sectional view through a container before pouringthe mixture of insulating material therein.

FIG. 2 is a section similar to FIG. 1, after the insulating material hasbeen poured.

As shown in the drawing the inner vessel 1 is adapted to receive theradioactive or like substance. A lead shield 2 surrounds the said vessel1 as shown. An outer metal casing 3 surrounds the lead shield 2 and isspaced therefrom, as shown, to form an annular space 4 around the leadshield 2, said outer metal casing 3 being provided with safety openings3a, as shown in FIG. 1. The insulating material mixture 5 according tothe invention is poured into the annular space 4 to fill said space withinsulating material 5 as shown in FIG. 2, and thereafter the openings 3aare closed by means of caps 6.

According to the present invention, the inter-wall insulation material 5is made by introducing a mixture of an aqueous suspension of aluminatrihydrate (A1 3H O) and particulate mild steel or iron into the space 4between the outer and inner walls 3 and 2 and allow ing the mixture toset. The water (of the suspension) oxidises the iron particles and theresulting reaction yields a hydrated iron sesquioxide (Fe O nI-I O); thefinal product is, therefore, a solid mass of a mixture of hydratedalumina and hydrated iron sesquioxide which has very good mechanicalstrength and good thermal conductivity.

Advantageously, an appropriate quantity of calcium sulphate hemihydrate(CaSO. VzH O) is-addcd to the mixture. The use of calcium sulphate orwet plaster as insulating material has already been suggested, forexample in US. Pat. No. 3,466,662 issued to Paul Blum on Sept. 9, 1969.However, adding wet plaster to the alumina trihydrate-particulate ironmixture apparently helps to achieve outstandingly good results, for asin the case of the Blum patent above mentioned, hydration of the calciumsulphate leads to homogeneous hardening of the mixture; also, theincrease in volume consequent upon the hardening or setting of theplaster helps to bond the inner lead shielding wall 2 to the inside ofthe outer metal casing 3. Also, because of the good contact thusproduced between the inner surface of the outer wall 3 and the outersurface of the inner wall 2, there is an appreciable improvement in heattransfer. More particularly, the setting of the plaster 5 leads to theformation of a crystal lattice which accelerates oxidation of the ironor steel powder and the formation of a very strong solid block.

In order to accelerate oxidation a flow of wet, and preferably warm, airis advantageously introduced into the space 4 between the outer andinner walls 3 and 2 before the normal safety openings 3a from this spaceare closed by caps 6. This flow of warm wet air can be followed by acurrent of dry warm air so as to at least partly dry the mixture, whichfinally has the form of a rigid porous block 5.

However, because of the presence of the hydrated alumina, an insulatingblock 5 of this kind contains a considerable quantity of water which inthe event of an accidental temperature rise (accidental outbreak offire) turns into steam, so that the lead shielding 2 cannot be damaged.Normal removal of the heat evolved by the radioactive material in thecontainer proceeds very satisfactorily during normal use of the package.Even after having lost its water of crystallisation in the form of steamthrough the safety openings 3a, the insulating material 5 according tothe invention retains sat isfactory mechanical strength and still has aheat con ductivity coefficient sufficient to ensure removal of the heatfrom the radioactive contents after such an accidental external fire.

The proportions by weight of a preferred starting mixture are, forexample, as follows:

Alumina trihydratc 35% Iron powder 25% Calcium sulphate 15% Water 25%The density of the resulting insulating material is about 2.2, althoughproperties of the product can be adjusted appreciably by variation ofthe proportions in the starting mixture. Other compounds adapted to forma crystal lattice which can support the other two ingredients of themixture can be used instead of calcium sulphate hemihydrate.

I claim:

1. In a double-walled container for radioactive and like substanceshaving an outer metal casing and a spaced inner wall of leadconstituting a radiation shield and, in the space between the outer andinner walls, an insulating material, the improvement wherein the insulating material comprises a solid mass containing at least 35 percent byweight of hydrated alumina.

6. The container as set forth in claim 1, wherein the insulatingmaterial also comprises approximately 15 percent of a compound adaptedto form a crystal lattice; and wherein the said solid mass also containshydrated iron sesquioxide.

7. The container as set forth in claim 6, wherein said compound iscalcium sulphate hemihydrate; and wherein the hydrated iron sesquioxidecontains at leat 25 percent by weight of iron powder.

2. The container set forth in claim 1, wherein the insulating materialalso comprises approximAtely 15 percent of a compound adapted to form acrystal lattice.
 3. The container set forth in claim 2, wherein saidcompound is calcium sulphate hemihydrate.
 4. The container as set forthin claim 1, wherein the said solid mass also contains hydrated ironsesquioxide.
 5. The container as set forth in claim 4, wherein thehydrated iron sesquioxide contains at least 25 percent by weight of ironpowder.
 6. The container as set forth in claim 1, wherein the insulatingmaterial also comprises approximately 15 percent of a compound adaptedto form a crystal lattice; and wherein the said solid mass also containshydrated iron sesquioxide.
 7. The container as set forth in claim 6,wherein said compound is calcium sulphate hemihydrate; and wherein thehydrated iron sesquioxide contains at leat 25 percent by weight of ironpowder.